Difference Between Diffusion and Osmosis

Also called as passive transport, diffusion is a type of transport process that allows small lipid-soluble (lipophilic) molecules to enter and leave the cell easily. This becomes possible because usually, the cellular membranes of living organisms are held by weak forces that allow these molecules to travel from the inside to the outside, and vice versa.

• One of the main drivers that make possible diffusion in cells is the difference in the concentration on each side of a biological membrane. This is called the concentration gradient^[1] or the chemical gradient.
• The movement of substances is from a region of high concentration (of that molecule or substance) to a region of relatively lower concentration.
• Diffusion does not require a semipermeable membrane.

Apparently, there are two types of diffusion^[2]: passive and facilitated.

• Passive diffusion is a type of diffusion that requires no protein channels during transports across the semipermeable membrane.
• On the other hand, facilitated diffusion is a type of diffusion that requires the presence of proteins (either carrier proteins or channel proteins) to occur. These proteins, however, do not use ATP in the process.

Diffusion affects almost every aspect of biology, making it one of organisms’ most important life processes. Living organisms use diffusion to facilitate the transport of substances within short distances. Aside from that, this process is very sustainable since no energy is required for it to happen.

Many diffusion processes occur in plants, one of which is during gas exchange. Plants utilize diffusion to facilitate the entry of carbon dioxide^[3] into the leaves for photosynthesis.

• First, the carbon dioxide molecules diffuse inward through the various pores in the leaves called stomata. After that, air spaces inside the leaves become filled with water vapor.
• This process highly depends on various environmental factors such as light and humidity.

Diffusion, too, occurs in animal cells in several ways–either to facilitate the entry of molecules into the cells or the exit of wastes out of them.

• For instance, during digestion^[4], food molecules like amino acids and sugars move in a concentration gradient to be delivered from the intestines into the blood-stream.
• Another example is the diffusion of waste products like urea from the body cells to the blood.

The definition of osmosis can vary depending on two perspectives. For one, osmosis is the movement of water from a lower solute concentration to one with a higher one. In other words, it can also mean the movement of water from a region with a higher water concentration to a lower concentration.

• It is important to note that osmosis only occurs across semipermeable membranes^[5] and is highly dependent upon the reduction of free energy of the solvents involved.
• Furthermore, it is also dependent on other factors like the solute potential, and the number of particles of solutes dissolved.

The main force regulating such process is called the osmotic pressure^[6]. Osmotic pressure is needed to stop the water flow across a membrane to separate the solutions with varying concentrations.

Check out the Osmosis simulation here.

Like diffusion, osmosis is highly important in living organisms as it facilitates the distribution of needed compounds and the exit of waste products out of cells. Without this process to regulate everything, substances essential for life enter or leave cells aimlessly, leading to potential biological failure.

The osmosis process is essential for vascular plants and is perhaps one of the main drivers for their evolution to land.

• The roots of these plants first absorb water and minerals from the soil, and the osmosis process allows them to move up the plant through the transport tissue xylem.
• Another osmotic process in plants is the transpiration of water molecules through their evaporation from the leaves.

For instance, one way is to concentrate the urine in humans. This will be initiated by the movement of water from the blood filtrate resulting in the formation of urine across the epithelial cell layer in the tubules (in the kidney) and ultimately into the blood.

• If this process does not occur by any chance, an average person will have to urinate several liters daily.

• As mentioned earlier, both diffusion and osmosis^[7] occur through passive transport. No additional energy is needed to drive them; they involve the movement of particles from a higher concentration to a lower one.
• Aside from those said earlier, one of the major differences is the type of medium where these processes happen. On the one hand, diffusion happens in any medium, be it solid, liquid, or gas, whereas osmosis can only happen in a liquid environment.
• Moreover, diffusion aims to attain equilibrium in the energy concentration in the biological system. Osmosis, on the other hand, requires the equilibrium of the concentration of solute among the intra-cellular environment and its outside environment.